This invention relates to protective coatings and particularly to industrial rollercoating applied thermosetting coatings exhibiting antiflashing properties.
Industrial rollercoating is a high speed coating process where paint is continuously applied by a combination of rollers to continuous steel or aluminum coil or sheets passing through the rollers. Line speeds often reach 900 feet per minute. Organic coatings typically exhibit an uneven film thickness when applied by direct or indirect rollcoat at conventional substrate speeds through the rollercoat. This uneven film weight occurs in random areas of the substrate causing an unacceptable discontinuous coating condition known as "flashing." Flashing can be so severe in some situations that a sheet coated at 20 milligrams per 4 square inch substrate surface can have flashed surface areas accumulating 4 milligrams per 4 square inch. The flashed surface areas have little or no paint coating applied to the affected surface areas and hence provides insufficient protection to the substrate. Flashing can be reduced, but not eliminated by considerably reducing the rollercoater line speed and/or adding major amounts of solvent to reduce the coating viscosity to the point of undesirable dripping (25 seconds #4 Ford cup). However, these corrective procedures are counter-productive in that high line speeds and high solids (low VOC) are highly desired.
It now has been found that a rollercoater process and composition can be substantially improved to avoid flashing by including in the coating composition dispersed additive amounts of a solid wax particle having a particle size less than about 50 microns to provide a non-solubilized dispersion of wax particles within the coating composition. In addition to avoiding flashing, higher solids and higher viscosity coatings can be utilized in accordance with this invention. These and other advantages of the invention are realized and will become more apparent by referring to the detailed description and illustrative examples.